182 STRUCTURAL BOTANY 



cells. Vaucheria therefore, though one of the simplest 

 of its order in vegetative respects, appears to rank 

 highest as regards reproduction. 



TYPE XIII. PLEUROCOCCUS VULGAEIS 



Before leaving the green Algae we will take one more 

 example, the very simplest we can find, as an illustra- 

 tion of a unicellular plant. Pleurococcus vulgaris is in 

 our climate perhaps the most abundant of all plants. 

 Everyone must have noticed' how commonly the trunks 

 of trees, palings, and wet walls are covered by a bright 

 green powdery layer, especially on the side away from 

 the sun. In damp winter weather the green coating 

 is most developed. This substance, though it may 

 include many different organisms, is chiefly made up of 

 Pleurococcus vulgaris. 



This Alga, which occurs in prodigious numbers, consists 

 of small rounded cells, sometimes quite separate, some- 

 times grouped together in little packets of two, four, or 

 eight. When adhering together, the sides in contact are 

 rather flattened. 



Each cell has a thin cellulose wall, and is densely 

 filled with protoplasm, which at first sight appears to 

 be coloured uniformly green. This, however, is not the 

 case, for the chlorophyll is really limited to definite 

 chloroplasts, of which there are usually several occupying 

 the outer part of the cell-contents. The rest of the 

 protoplasm is colourless. About the middle of the 

 cell is a nucleus, containing a nucleolus (Fig. 77). 

 The cells divide freely into two; successive divisions 

 take place in all three directions, and are at right angles 



